Porous calcium phosphate ceramics prepared by coating polyurethane foams with calcium phosphate cements

Abstract

Porous calcium phosphates have important biomedical applications such as bone defect fillers, tissue engineering scaffolds and drug delivery systems. While a number of methods to produce the porous calcium phosphate ceramics have been reported, this study aimed to develop a new fabrication method. The new method involved the use of polyurethane foams to produce highly porous calcium phosphate cements (CPCs). By firing the porous CPCs at 1200 °C, the polyurethane foams were burnt off and the CPCs prepared at room temperature were converted into sintered porous hydroxyapatite (HA)-based calcium phosphate ceramics. The sintered porous calcium phosphate ceramics could then be coated with a layer of the CPC at room temperature, resulting in high porosity, high pore interconnectivity and controlled pore size.

Introduction

Calcium phosphate ceramics include a variety of ceramics such as hydroxyapatite (HA), tricalcium phosphate (TCP), calcium phosphate cement (CPC), etc. These ceramics have excellent biocompatibility and bone bonding or bone regeneration properties. They have been widely used in no- or low-load-bearing applications [1], [2]. In orthopedic surgery, they are used for filling bone defects as a result of the removal of diseased or damaged bones. In dentistry, calcium phosphate ceramics are used for the augmentation of deficient mandibular of maxillar ridges. Dense or porous calcium phosphate ceramic coatings are often applied on strong and load-bearing core materials for biological fixation or osteointegration of load-bearing implants such as hip stems and dental roots. Porous calcium phosphate ceramics are also expected to play important roles in treating bone problems with the emerging tissue engineering approach, as it involves loading proper cells into porous ceramics (scaffolds) and implanting the cell-loaded scaffolds into a host body for achieving bone tissue regeneration. In fact, a variety of porous ceramics have been investigated for the delivery of drugs, marrow and cultured marrow cells, namely, HA [3], TCP [4], biphasic HA/TCP [5] and calcium phosphate cements (CPCs) [6].

A number of papers have reported the methods for the preparation of useful porous scaffolds. The earliest study could be the fabrication of porous HA ceramics by duplicating the macroporous structure of natural ocean corals [7], [8]. Liu [9], [10] used polyvinyl butyral (PVB) particles as a pore former to prepare porosity-controlled HA ceramics through both a solid process and a liquid process.

Sepulveda et al. [11] produced open-cell hydroxyapatite foams through the technique of gelcasting. Porous hydroxyaptite ceramics were also produced by impregnating porous polyurethane foams with a slurry containing HA powder, water and additives [12], [13]. Milosevski et al. [14] produced porous tricalcium phosphate with a porosity of 55–70% using a polyurethane foam.

While CPCs have low mechanical strengths like other calcium phosphates, they are reported to be biodegradable or actively remodeled in vivo [15]. Furthermore, due to the low temperature involved during the setting of the cement, proteins or drugs can be incorporated into the matrix of the cement. In addition to dense CPCs, there has been significant research on the macroporous CPCs. Yoshikawa et al. [16] made porous CPC scaffolds with the addition of sucrose into the CPC paste of the powder and the liquid components. The sucrose in the cement was then removed by boiling the cement in water to produce porosity. Markovic et al. [17] reported the formation of CPC with 11% macroporosity by the addition of mannitol to the cement during mixing with water for 20 h. Takagi and Chow [18] used a number of water-soluble pore formers to prepare CPCs with up to 50% microporosity. More recently, Barralet et al. [19] prepared macroporous CPCs using a mixture of frozen sodium phosphate solution particles and CPC powder. After compacting the mixture, the frozen sodium phosphate particles were allowed to thaw to set the cement and create the porosity. In addition, Del Real et al. [20] developed a new way to create macropores in calcium phosphate cements. The method involved adding NaHCO₃ to the starting cement powder and using an acid liquid to obtain CO₂ bubbles for the generation of macropores.

The purpose of this study was to demonstrate the feasibility of preparing porous calcium phosphate ceramics by coating polyurethane foams with a calcium phosphate cement, followed by firing the calcium phosphate cement at a high temperature. The fired porous calcium phosphate ceramics were then coated with the calcium phosphate cement again at room temperature. The advantage of this method was the achievement of high porosity, high pore interconnectivity and controllable pore size. The method resulted in porous bioceramics without the problem of residual pore formers, in contrast to those methods involving the incomplete leaching of the pore formers such as sugars and salts. By coating the porous calcium phosphate ceramics with the calcium phosphate cement, the porous calcium phosphate ceramics maintained the capability of incorporation of drugs or proteins.